The present invention relates to a method for controlling an AC induction motor. More particularly, the invention relates to a method for controlling an AC induction motor whereby the motor is controlled so as to make the torque response of the AC induction motor behave substantially the same as a DC motor.
A method for controlling an AC induction motor which enables the motor to be controlled in such a manner has been described by F. Blaschke, "The Principle of Field Orientation as Applied to the New TRANSVEKTOR Closed-Loop Control System for Rotating-Field Machines", Siemens Review, Vol. XXXIX, No. 5, pp. 217-220 (1972). With reference to FIG. 1, the basic principle of field oriented control is to control both the phase and the amplitude of the stator current so that the field component of the current remains constant and the torque component is controlled according to the torque command. In such a control method, the three-phase stator currents are transformed into a D-Q reference frame which is rotating at an angular speed .omega., that is, at the frequency of the applied power source. In FIG. 1, I.sub.D is termed the field component current, and I.sub.Q the torque component current. If .vertline.I.sub.D .vertline. is held constant and I.sub.Q controlled in accordance with the torque command, the induction motor can be controlled so as to operate in the same manner as an armature-controlled DC motor with a fixed field oxcitation. The d-q coordinate system in FIG. 1 is fixed respect to the stator; that is, the d-q coordinate system is a stationary reference coordinate system.
FIG. 2 is a block diagram showing how the field oriented control method can be implemented. The control system receives as inputs a torque command T* and a flux command .0.*. From these values and from a value I.sub.f derived from the sensed flux .PSI. in the rotor gap of the motor 30, a current command calculator 10 calculates values of I.sub.d * and I.sub.q *, where the asterisks indicate command values. In this diagram, L.sub.2 represents the self inductance of the rotor of the motor 30, r.sub.2 the resistance of the rotor and L.sub.m the magnetization inductance of the motor. From I.sub.D * and I.sub.Q *, a transform from the D-Q (rotating) to the d-q (fixed) coordinate system is effected utilizing the value of I.sub.f calculated from the sensed flux .PSI.. A second transform from the d-q coordinate system to the three-phase a-b-c system is then effected to derive values of i.sub.a *, i.sub.b * and i.sub.c * representing the three-phase currents flowing in the stator windings of the motor. To drive the motor 30 with voltages, the current commands i.sub.a *, i.sub.b * and i.sub.c * are changed by an inverter 20 into voltages v.sub.a, v.sub.b and v.sub.c which are applied to the stator coils of the motor 30.
Although this scheme is successful in attaining the desired object of driving an AC induction motor so that the output torque behavior is similar to that of a separately excited DC motor, the implementation required therefor is complex and costly. Specifically, it is difficult to provide a flux sensor inside the motor, as the scheme unavoidably requires. Also, two separate transforms are required: that from the D-Q coordinate system to the d-q system and that from the d-q system to the a-b-c system, which transforms are complex to implement and time consuming to execute in operation.
Accordingly, it is desirable to provide a method for controlling an AC induction motor in which no flux sensor is required in the motor and which can be implemented in a manner simpler than that required for field oriented control as described above, specifically, without the need for coordinate transforms as required for field oriented control.
It has been reported by S. Yamamura and S. Nakagawa in "Equivalent Circuit and Field Acceleration Method of A-C Servomotor by Means of Induction Motor", Trans. B. IEE of Japan, Vol. 102-B, p. 439 (1982) that, with respect to a single-phase equivalent circuit of an induction motor, if an input voltage V.sub.1 is controlled in such a manner that an exciting current I.sub.0 is held constant in amplitude and continuous in phase when a transient occurs in an input torque command T*, then there will be no torque transient and the motor can be controlled so as to provide a torque response similar to that of a separately excited DC motor. This paper gives several different equivalent circuits of AC induction motors, including a symmetrical-T type, an asymmetrical -T-I type, an asymmetrical-T-II type and an L type, which are all well known per se from other studies. The paper concludes that this principle is generally applicable to all of these equivalent circuits. The Yamamura and Nakagawa paper does not, however, disclose any way of implementing a control system for an AC induction motor which operates using this principle. Moreover, the paper offers no proof of the assertion that, for all the different equivalent circuits given, if the exciting current is controlled in the manner described, there will be no torque transient and the motor can be controlled so as to provide a torque response the same as that of a separately secited DC motor.
Thus, it is a further object of the present invention to determine for which if any of the different equivalent circuits disclosed by Yamamura and Nakagawa the assertion of controllability in the manner of a DC motor is true and to provide a method for implementing control of an AC induction motor accordingly.